1. Field of the Invention
The invention relates to a television signal receiver and in particular to a television signal receiver capable of cancelling linear and non-linear distortion.
2. Description of the Related Art
Group delay distortion commonly existing in television signal receiver decreases signal quality, and needs to be cancelled. Please refer to FIG. 1. FIG. 1 shows a block diagram of a conventional television signal receiver 100. The television signal receiver 100 includes a tuner 110, an intermediate frequency (IF) filter 120, an intermediate frequency (IF) demodulator 130, a baseband distortion canceller 140, and a controller 150. The receiver 100 receives a radio frequency (RF) television signal SRF from a transmitter (not shown). The tuner 110 selects one channel from the RF television signal SRF, and down-converts the selected channel from RF band to IF band. The IF filter 120 filters the selected channel to generate an IF filtered signal SIF. The IF demodulator 130 demodulates the IF filtered signal SIF to generate a baseband signal SBB. The baseband distortion canceller 140 cancels a group delay distortion in the baseband signal SBB to output a distortion-cancelled signal Scanc. After cancelling the group delay distortion, the distortion-cancelled signal Scanc meets the requirement of a flat group delay response. Finally, the distortion-cancelled signal Scanc is sent to the television. Additionally, a controller 150 generates a control signal Sctrl to program the baseband distortion canceller 140 to cancel the group delay distortion. A detailed description of cancelling the group delay distortion is provided in the following.
Please refer to FIG. 2. FIG. 2 shows a plurality of curves of the group delay response. The baseband distortion canceller 140 provides programmable group delay (such as curves GDI and GDII) to cancel the group delay distortion of the receiver circuitry 100. After cancellation, the group delay response of the receiver circuitry becomes flat (see the curve GDcanc). Ideally, the group delay response of the receiver circuitry 100 should be flat (the curve of which is similar to curve GDcanc) and hence the baseband distortion canceller 140 would be unnecessary. In practice, due to the various group delay precorrection transmitted from various transmitters and the deviation of IF filter group delay of various IF filters 120, the group delay response of the receiver circuitry 100 is not flat. The above-mentioned group delay precorrection is performed at the transmitter. Various broadcasters may send the RF television signals with various group delay precorrections (e.g. half, full, or no precorrection). The above-mentioned IF filter group delay is a characteristic of the IF filter (filter 120) at the receiver. Various IF filters may have various IF filter group delays. An adequate IF filter has an almost flat IF filter group delay while an inadequate IF filter has an uneven IF filter group delay. Therefore, the baseband distortion canceller 140 must provide various programmable group delay (such as curves GDI and GDII) to cancel various group delay precorrections and IF filter group delays in different situations.
As one can see, the conventional baseband distortion canceller is capable of cancelling the group delay distortion in the baseband stage. As to distortion characteristic, the group delay distortion is a type of linear distortion, and the characteristic of which is substantially invariant in each stage (RF, IF, and baseband) of the receiver circuitry. The characteristic of non-linear distortion, however, changes in different stages. In other words, the non-linear distortion is hard to be processed and cancelled in the baseband stage since the characteristic is already changed in the demodulation process performed in the IF demodulator. Hence, the conventional baseband distortion canceller is hard to cancel the non-linear distortion.